Micro slit machining using EDM with amodified rotary disk electrode(RDE) H.M. Chow , B.H. Yan , F.Y. Huang Department of Mechanical Engineering, National Central University, Chung-Li, 32054, Taiwan, ROC Name ： Wen-Chen Huang ID ： M9710108 Date ： 2009/5/19 1
Abstract• The effects of polarity, discharge current, pulse duration and rotational speed on the material removal rate (MRR), the electrode wear rate (EWR), the expansion of slit, the surface profile and the recast layer of micro slit machining are reported and discussed. 3
Introduction• MEDM equipment is too expensive to be able to be used widely.• WEDM suffers from the breakage susceptibility of the superfine wire.• This new application of RDE-EDM machining is achieved by locating the rotating disk electrode below the workpiece to improve the debris removal rate.• The benefits of this modified RDE-EDM also include the obtain of an improved EDMed circuit system that reduces the discharge current, and the offering of a compact designation to stabilize RDE vibration during machining. 4
Experimental procedure • The modified RDE-EDM developed in this study consists of a modified non-micro EDM machine (a die-sinking EDM) with a RDE.Fig. 1. Schematic diagrams of EDM with: (a) a conventional RDE (b) a modified RDE. Note that the relative position of the workpiece and the RDE is reversed in the modified RDE-EDM. 5
Experimental procedureThe RDE-EDM experimental conditionsConditionsElectrode size Cu, D=ø42 mm, t=25, 50, 75, 100 μmWorkpiece Ti–6Al–4V, t=0.45 mmPolarity Negative ( － ), positive ( ＋ )Dielectric KerosenePeak current Ip (A) 0.06, 0.1, 0.5High voltage (V) 280Gap voltage (V) 25Duty factor 0.55Pulse duration τp (μs) 2, 5, 10, 20Working time (min) 4Revolutions of electrode (rpm) 0, 10, 20, 50, 150Target depth 1.02mm 6
Experimental procedureFig. 2. A detailed schematic diagram of the modified RDE-EDM proposed in this study. 7
Results and discussionFig. 3. The material removal depth vs. Fig. 4. The electrode wear vs. the rpm the rpm of the RDE electrode of the RDE electrode with the with the discharge current as a discharge current as a parameter. parameter. 8
Results and discussion• (Fig. 3)The former was because the relative motion between the electrode and the workpiece increased the debris removal rate, whilst latter might be due to the large centrifugal force at high rotational speed that made it difficult for the dielectric fluid to flow into the gap, thus decreasing the discharge activity.• The workpiece was located at the top of the RDE in the present modified RDE-EDM, thus the debris removal mechanism was increased not only by the rotating electrode but also by the gravity of the debris itself. 9
Results and discussionFig. 5. The effects of electrode thickness on the material removal depth, the expansion of the slit, and the electrode wear. 10
Results and discussion• At a discharge current Ip = 0.1A, the discharge density was too high for an EDM process to be stable.• An optimized discharge density could be reached by using a thicker electrode, the optimized arrangement possibly allowing the use of a greater material removal depth and therefore resulting in less electrode wear. 11
Results and discussionFig. 6. The material removal depth vs. Fig. 7. The expansion of the slit vs. the pulse duration for both the pulse duration for both negative discharge polarity and negative discharge polarity and positive discharge polarity. positive discharge polarity. 12
Results and discussion• A higher MRR was observed with adopting the RDE-EDM as the cathode.• However, in handling the positive-polarity condition, the dissociated carbons element in the dielectric fluid tend to adhere to the anode (Ti alloy), which may form a TiC recast layer by solid solubilization and then diffuse gradually during sample melting and solidification in the EDM process.• This phenomenon may, somehow, reduce the material removal rate. Furthermore, the melting point of TiC (3150°C) is about twice that of Ti (1660°C). It is more desirable to adopt a negative polarity in a acquiring low EWR and a high MRR. 13 This practice is adopted in the present work.
Results and discussionFig. 8. Cross-sectional SEMs of micro slits obtained by both positive and negative discharge polarities for: (a) the outlook of the slit; (b) the 14 bottom of the slit, and; (c) the surface of the slit.
Results and discussion• (a)The depth of the slit was twice the depth with negative polarity than it was with positive polarity.• (b)The thermal effect area was smaller and the recast layer was thinner with negative polarity.• (c)More sub-crack surfaces are observed with positive polarity, which is consistent with the lower MRR associated with positive polarity.• The deposit carbon reacts with Ti to form TiC which has a high melting point above 3150°C and requires a greater energy density to be removed with positive polarity: this also accounts for lower MRR with positive polarity. 15
Results and discussionFig. 9. The material removal depth vs. Fig. 10. The expansion of the slit vs. the pulse duration with discharge pulse duration with discharge current as a parameter (the current as a parameter ( the negative discharge polarity is negative discharge polarity is adopted). adopted). 16
Results and discussion• Too-long a pulse duration (>6 μs) or too-high a discharge current (0.5 A) result only in a lower removal rate and worse surface conditions.• Only a small slit expansion was obtained at the low discharge current of 0.06 A and a pulse duration of 2–5 μs with negative polarity. 17
Results and discussionFig. 11. A photograph of a single-slit Fig. 12. Photograph of a multiple-slit microstructure. The micro slit microstructure (with 10 slits)18 is compared with a hair.
Results and discussion• The resultant width of the slit was 42 mm, and the depth was 1.02 mm• The tolerance of the slit width and slit depth is ±1 μm, and ± 5 μm, respectively.• The uniform wear in the radial direction was reduced to 0.02 mm after the carrying out of the machining of the 10-slit microstructure process. 19
Conclusions1. The modified RDE-EDM can improve MRR by locating the workpiece above the RDE. EWR also decreases uniformly around the periphery of the disk electrode with this modified arrangement. The position accuracy and vibrational stability of RDE are improved over those of classical RDE-EDM to achieve a high standard of micro slit machining. 20
Conclusions2. Although Ti is known to be a difficult material to cut, an MRR as high as 1.5 mm3 /min-1 is demonstrated in this study with the modified RDE- EDM, using the optimum working condition at 10– 20 rpm, a discharge current of 0.1 A, and a pulse duration of 5 μs.3. Optimized discharge current is essential because the temperature during discharge is extremely sensitive to the discharge current due to the small area of the micro slit. A greater MRR and lower EWR can be obtained by properly optimizing the discharge current. 21
Conclusions4. Negative polarity for the workpiece was adopted for the present micro machining. A greater MMR and lower EWR in the machining of the Ti alloy process was observed under such working polarity.5. The finished surface of the slit shows less cracking, less recast layer and a smaller expansion of the slit with negative polarity which later is recommended for further work in this and similar fields. However, the cracking, the recast layer, and the expansion of the slit all increase as pulse duration increase. 22
Conclusions6. The best working conditions are: Ip = 0.06 A; τp = 2 μm, and; 20 rpm to obtain the smallest slit width in these experiments. However the optimum conditions may be different when applied to other EDM processes. A preliminary calibration of each EDM process to acquire the optimization is therefore essential in applying this new technique. 23